This application claims priority under 35 USCxc2xa7119 and contains subject matter related to Japanese Patent Application No. 11-197284 filed in the Japanese Patent Office on Jul. 12, 1999, the entire contents of which are hereby incorporated by reference herein.
1. Field of the Invention
The present invention relates to an electrophotographic apparatus for use in a copying machine, a facsimile, a laser beam printer, or the like, that forms an image by visualizing an electrostatic latent image with a two-component developer containing a mixture of toner and carrier.
2. Discussion of the Background
In a known electrophotographic apparatus, to form a toner image on a recording member such as a transfer sheet, first an electrostatic latent image is formed on a surface of a photoreceptor including a photoconductive element serving as an image bearing member by exposing the surface of the photoreceptor which is uniformly charged. Subsequently, the electrostatic latent image on the photoreceptor is developed with toner, and then the toner image is transferred to the recording member. The transferred toner image is fixed on the recording member under the influence of heat and pressure by a fusing device.
In the above-described electrophotographic apparatus, the toner which is not transferred to the recording member and remains on the surface of the photoreceptor is collected by a cleaning device such as a blade, a brush, etc. Then, the collected toner is kept is a container and disposed or returned to a developing device for reuse in subsequent development. The collected toner typically includes a paper powder (paper dust) which is transferred to the surface of the photoreceptor from a transfer sheet at a transfer sheet. The paper powder on the surface of the photoreceptor is also collected by the cleaning device together with the toner.
With regard to the disposal of toner, there arise some problems. First, the disposal of the waste toner needs to be carried out regularly. Second, a storage space for the collected toner is required, so that the size of the electrophotographic apparatus increases. Third, the disposal of toner is generally contrary to the social demand for reducing waste.
For the above-described reasons, in the electrophotographic apparatus, a demand for recycling toner has increased with increased environmental sensitivity, and a cost reduction has been desired, such as by decreasing the size of the electrophotographic apparatus and extending the useful life of toner. In order to fulfill these demands, a method is employed wherein residual toner on the photoreceptor is collected by a cleaning blade after the toner image on the photoreceptor is transferred to the transfer sheet and is returned to the developing device for recycling.
However, when the toner is recycled in the electrophotographic apparatus, a deterioration of image quality may be caused by the paper powder included in the recycled toner. Specifically, when paper powder of relatively large particle size is attached to a non-image portion, the fiber of the paper powder is likely to be recognized by the human eye on a transfer sheet. Moreover, some paper powders are charged with an opposite polarity to that of the toner particle, and toner particles are likely to attach to the paper powders of opposite polarity. When the paper powder with toner particles is attached to a non-image portion, adhering of toner becomes conspicuous on the transfer sheet.
Typically, the above-described deterioration of image quality has not occurred in a background electrophotographic apparatus using recycled toner. The background electrophotographic apparatus has been configured such that the recycled toner is returned to a reservoir called a toner hopper and is mixed with new toner while being agitated therein. Subsequently, the mixture of the recycled toner and new toner is supplied to the developing device after passing through slit openings of a slitter. Thus, the toner from which the paper powder is removed by the slitter can be supplied to the developing device. However, the above-described reservoir, such as a toner hopper, has not been in great demand from the viewpoint of compact design of the apparatus. Therefore, the configuration of the electrophotographic apparatus has been changed such that the recycled toner is returned directly to the developing device from the cleaning device. A reference will be made to this type of background electrophotographic apparatus referring to FIGS. 5 and 6.
FIG. 5 is a schematic view illustrating a construction of the background electrophotographic apparatus, and FIG. 6 is a schematic view for explaining a behavior of toner in a developing device of the electrophotographic apparatus. In the electrophotographic apparatus of FIG. 5, a cylindrically shaped photoreceptor 41 as an image bearing member is supported so as to rotate in the direction indicated by an arrow. Arranged around the photoreceptor 41, are a charging roller 42, an exposing device 43, a developing device 44, a transfer belt 46, a cleaning blade 47, a rotating blade 48, and a toner returning coil 49. The above-described elements except the transfer belt 46 are housed in a photoreceptor/cleaning unit 50 (hereinafter referred to as a PCU).
The developing device 44 is housed in a case having an opening. At the opening, a developing sleeve 45 is rotatably supported and disposed opposite the surface of the photoreceptor 41. A paddle 54 is rotatably supported and disposed at the position opposite the developing sleeve 45 in the case. On the periphery of the paddle 54, spirals 52 are spirally provided. In the paddle 54, a screw conveyor 53 is provided and supported such that the screw conveyor 53 rotates in the same direction as the paddle 54.
Referring to FIG. 6, new toner and recycled toner enter the developing device 44 from a toner supply opening 51, and then enter the paddle 54 from a toner inlet 56. Subsequently, both new toner and recycled toner are mixed with carrier in the paddle 54, and then are conveyed by the screw conveyor 53 as a two-component developer in the direction indicated by arrow A while being agitated. Further, the two-component developer is discharged from the paddle 54 through a developer outlet 55, and then is conveyed by the spirals 52 in the direction indicated by arrow B.
In the above-described background electrophotographic apparatus, when image forming operations are performed for a long term, the toner particles attach to the paper powder which has opposite polarity to that of the toner particles, and thereby the adhering of toner at a non-image portion typically occurs. Moreover, the fiber of the paper powder of relatively large size becomes conspicuous on the non-image portion of the transfer sheet.
The present invention has been made in view of the above-discussed and other problems, and an object of the present invention is to address these problems.
Accordingly, a further object of the present invention is to provide a novel electrophotographic apparatus including a toner recycling mechanism that can obtain an appropriate image without deterioration of image quality in a compact configuration.
These objects and others are achieved according to the present invention by providing a novel electrophotographic apparatus which forms an electrostatic latent image on a photoreceptor and includes a developing device configured to accommodate a two-component developer including toner and carrier and to develop the electrostatic latent image with the developer. The toner which has been used for developing the electrostatic latent image is recycled to the developing device so as to be used again. The developing device includes a developer conveying path so as to convey the recycled toner therethrough. A screening member having openings to screen the recycled toner is arranged in the developer conveying path.
Preferably, the size of the openings of the screening member is greater than twice a volume mean particle diameter of the carrier and smaller than a size of an opening of 10 mesh.
The developing device may further include a rubbing member which rubs against the screening member.
A size of the screening member may be 10 mm2 or more.